1. Field of the Invention
The present invention relates to ink jet printing systems for printing rasterized pixel data. In particular, the present invention relates to an ink jet printing system using multiple printing passes to print rasterized pixel data at a resolution greater than an available ink jet nozzle resolution.
2. Description of the Related Art
Conventional ink jet printing systems utilize ink jet print heads for printing pixels upon a recording medium. Ink jet print heads contain ink jet nozzles, which eject ink droplets onto the recording medium during a printing pass over the recording medium. Ink jet nozzles are separated from each other on an ink jet print head by a fixed vertical distance. The fixed vertical distance determines a nozzle resolution, which is the maximum vertical pixel resolution that can be printed by the ink jet print head in one printing pass. Accordingly, conventional printing systems cannot print pixels in one printing pass at a resolution greater than the nozzle resolution.
In addition, conventional ink jet systems often utilize multiple printing passes to reduce "banding" within printed image data. Banding usually occurs when, for example, rasters 1 to 24 are printed, followed by rasters 25 to 48, and so on. As a result, gaps may appear between rasters 24 and 25. Conversely, printed rasters 24 and 25 may overlap due to misalignment, resulting in a dark region at the area of overlap. "Multi-pass" printing attempts to address this problem by printing pixels of each raster using at least two different nozzles.
More specifically, multi-pass printing systems utilize a multi-pass mask to identify pixel data to be printed on a first printing pass and to identify pixel data to be printed on a second printing pass. Printed data resulting from such a mask is shown in FIG. 1. For discussion hereinbelow, the printed data in FIG. 1 is shown superimposed onto thirty-six 1/720 inch.times.1/720 inch square regions.
With reference to FIG. 1A, pixels centered around a numeral "1" were printed during a first printing pass, and pixels centered around a numeral "2" were printed during a second printing pass. Moreover, the numerals are placed at the exact positions toward which their respective ink droplets were ejected. Accordingly, after converting input pixel data to an available nozzle resolution, conventional systems apply a mask to the data in order to indicate pixels which should be printed during a first printing pass, and apply a second mask to the data to indicate pixels which should be printed during a second pass.
Printed output resulting from multi-pass masking is significantly affected by horizontal misalignment between the first and second printing passes. For example, FIG. 1B shows printed output on a recording medium resulting from multi-pass masking in which a one pixel horizontal misalignment exists between the first and second printing passes. As shown, pixel coverage over the recording medium suffers. Moreover, FIG. 1C shows a two pixel misalignment between pixels of a first pass and a second pass. As shown, ink coverage of the recording medium is greatly decreased, in FIG. 1C, from the ideal situation illustrated in FIG. 1A.
Accordingly, what is needed is a system for printing rasterized data at a vertical resolution greater than an available ink jet nozzle resolution, and in which print output is less susceptible to horizontal misalignment than in conventional systems.